4. Pulse of the Sun
Taking the Sun's pulse
We can look inside the Sun by observing the slow, rhythmic, in-and-out motions of the photosphere. These widespread throbbing oscillations are caused by internal sound waves. When the sounds strike the photosphere and rebound back down, they disturb the gases there, causing them to move in and out with a period of about five minutes.
The sound waves are trapped inside the Sun and cannot travel through the near vacuum of space. Nevertheless, when these sounds propagate upward to the photosphere they disturb the gases there and cause them to rise and fall, producing widespread throbbing motions (Fig. 4.1). These vertical oscillations can be tens of thousands of meters high and travel a few hundred meters per second. Such movements are imperceptible to the eye, but sensitive instruments on the ground and in space routinely pick them out. They are detected as tiny, periodic changes in the wavelength of a well-defined spectral line, or as miniscule variations in the Sunís total light output.
When oscillations move part of the photosphere toward Earth, the wavelength of light emitted from that region becomes shorter, the wave fronts or crests appear closer together, and the light therefore becomes bluer. This shift occurs because each successive wave has a shorter distance to travel than the one before it did in order to reach Earth, so the distance between waves, the wavelength, becomes shorter. When the oscillations carry localized regions away from Earth, the wavelength becomes longer and the light redder. Each wave has farther to travel than the one before it did. The magnitude of the wavelength change, in either direction, establishes the velocity of motion along the line of sight, which is called the radial velocity.
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Copyright 2010, Professor Kenneth R. Lang, Tufts University